1. Field of the Invention
This invention relates to binary signal detection circuits and more specifically to improvements therein for improving the signal to noise ratio and limiting the pulse widths of the detected signal. The invention is useful for various signal detection applications and particularly desirable for use in magnetic readback systems for high density magnetically recorded data.
2. Description of the Prior Art
Magnetic recording of binary data is customarily performed by effecting polarization changes or transitions in a magnetic storage medium. In order to increase the storage capacity for a given storage medium it becomes necessary to increase the data packing density, that is, the number of information bits recorded along a given track in a storage medium. To achieve this greater packing density, magnetic recording of the data is generally carried out by means of various encoding techniques that have been developed for the purpose of reducing the number of transitions per bit, or for a group of bits, while simultaneously assuring that the maximum interval between transitions remains short enough so that a self clocking capability can be maintained for the recovery of the data when reading from the storage medium. Nevertheless, if the transitions become too closely spaced as a consequence of unduly high packing density, so called pulse crowding occurs because of inherit features of the magnetic recording and reading process. Pulse crowding is manifested in the course of reading from the storage medium by interference between read pulses arising from adjacent transitions. This interference occurs because the read pulses overlap in time to some degree when the transitions are too closely spaced and it is aggravated by asymmetry and phase distortion of the read pulses. The phase distortion results from differential phase shifts of the constituent frequency components of the signal which cause broadening of the individual pulses. It is well known in the magnetic data storage art, however, that the read pulses supplied from a magnetic read head typically contain such distortion and accordingly various techniques have been developed in the art to provide compensation therefor. Suitable phase shift compensation may be provided, for example, by the use of phase equalization means as disclosed in U.S. Pat. No. 3,405,403 issued Oct. 8, 1968 to G. V. Jacoby, et al. As becomes apparent, peak shift is undesirable inasmuch as the peaks are representative of the data transitions recorded on the storage medium. In any event, even in the absence of such distortion and asymmetry or the provision of compensation therefor, if the read pulses overlap due to pulse crowding, interference will occur between adjacent pulses causing variable amplitude and shifting of the peaks of the read signal. The foregoing shifting of pulses, so called bit shift or peak shift, causes resultant errors either as a consequence of failure to detect a transition indicative of a data bit or false interpretation of noise in the read signal as representative of a data bit. For accurate signal detection the relative time occurrence of the peaks must be preserved in order to recover the data. It is, therefore, common practice in the magnetic data storage art to provide some sort of compensation or equalization which acts to narrow the widths of the individual read pulses so that they do not appreciably overlap and thus do not cause intolerable peak shift or amplitude variation of the read signal. The present invention is concerned with read signal equalization. Hence, the remaining consideration of the prior art section of this description will be limited generally to that technique and discussed ultimately in relation to a particular signal detector circuit which is improved by means of the principles of the present invention.
As mentioned above, data recovery pursuant to reading from a magnetic storage medium is typically performed by sensing the occurrence of peaks of the read signal, and it is for that reason that provision is made for narrowing the read pulses so as to preclude interference therebetween which otherwise might intolerably shift the peaks. Additional factors attendant to pulse narrowing and which affect data recovery must also be considered. For instance, the more a pulse is narrowed the greater its bandwidth becomes thus requiring a substantially commensurate increase of the read system bandwidth with an accompanying increase in noise. This is undesirable because noise which is at or near the peak of a read pulse can act to shift the peak of the pulse, so called noise induced peak shift. It is, therefore, important to provide equalization which appropriately acts on the relatively broad read pulses so as to provide a degree of pulse narrowing sufficient to eliminate or at least substantially reduce interpulse interference, whereby peak amplitude variation and bit shift are satisfactorily avoided. But the pulse narrowing must not be so great as to substantially increase noise in the read system. To obtain this result, the individual read pulses should be narrowed so as to have a contained amplitude spectrum, that is, a spectrum of limited frequency range. The point is that since bit shift can be cause by both interpulse interference and noise, a trade off should be made with regard to pulse narrowing which has the effect of advantageously reducing interpulse interference but which unfortunately enhances noise.
A typical equalizer circuit is disclosed in U.S. Pat. No. 4,081,756 issued Mar. 28, 1978 to Price et al and assigned to the assignee of the instant invention. The preferred form of the equalizer disclosed therein provides even-function and preferably substantially cosine-forth-power amplitude spectrum shifting. In other words, the equalizer responds to an isolated time domain pulse input so as to provide isolated a time domain pulse output having a substantially cosine-fourth-power amplitude spectrum in the frequency domain. With even-function amplitude spectrum shaping and other so called forced amplitude spectrum output shaping techniques, each shaping technique serves to compensate the variable peak amplitude input signal which is derived by the transducer and is applied to the equalizer so as to transform the input signal into a constant peak amplitude output signal having narrowed pulses and a prescribed amplitude spectrum of limited frequency range. It should be understood that all such read signals have data and noise components. The forced amplitude spectrum output shaping technique has inherent in its process predetermined the desirable output shape to be produced by the equalizer in response to the read signal. However, the noise component of the desired (output shape) signal can be altered in only a limited way, that is, by pulse narrowing wherein the relatively broad read pulses are narrowed sufficiently to at least substantially reduce interpulse interference. But the pulse narrowing is dictated by the desired output shape and must not be so great as to unduly increase noise in the read system and thus produce noise induced peak shifts. For a pulse type signal, the signal to noise ratio is a quantitative measure of a signal having data and noise components. The maximum signal to noise ratio for pulses produced by forced amplitude spectrum output shaping is inherently limited by the predetermined shape such as a cosine-fourth power spectrum for the output pulses made in response to the input read signal. Therefore, although the equalizer described above is quite useful there is a need for a better equalization by improving the signal to noise ratio of the read signal in order that the pulse peaks representative of the data may be detected with less uncertainty.